Thermionic cathode



March 25, 969 R. J. BONDLEY 3,434,812

THERMIONIC CATHODE Filed Sept. 8. 1966 INVENTORI RALPH J. BONDLEY,

BY HIS TTORNEY.

United States Patent US. Cl. 29-182.5 Claims ABSTRACT OF THE DISCLOSUREAn improved dispenser cathode having an emission density of the order ofa./cm. at 1000" C. consisting essentially of a refractory metal matrixincluding 511% by weight barium-strontium-tungstate Ba Sr(WO Thisapplication is a continuation-in-part application of copendingapplication Ser. No. 360,439 filed Apr. 16, 1964, now abandoned, andassigned to the same assignee as the present invention.

This invention relates to improved dispenser cathodes and moreparticularly to an improved high emission density thermionic cathode anda prescribed process of combining and activating certain specificallyprecribed ingredients to obtain increased high emission densitycharacteristics.

In various electronic applications generally, and particularly withrespect to high power output beam devices operative in the ultra highfrequency and microwave frequency ranges, a dispenser cathode plays anextremely important if not limiting part for increased power output.Increased power output in many of the frequency generation devices maybe obtained by means of thermionic cathodes capable of emitting a stableD-C current of high density over a long period of time withcomparatively small heater power consumption. There is an everincreasing need, for example, for hot cathode devices which are capableof operating over many hundreds of hours of continuous operation in thetemperature ranges of 900 to 1200 C. and which will, under theseconditions, provide an emission current density of in excess of at least5 amperes per square centimeter. Such a dispenser cathode as described,when employed in a high frequency generator depending upon a high powerelectron beam, will provide a substantial increase in the power outputof the generator in addition to the overall increase of efliciencythereof.

Accordingly, it is an object of this invention to provide an improvedthermionic dispenser cathode.

It is another object of this invention to provide an improved process toyield an increased emission density cathode.

It is yet another object of this invention to provide a continuous highemission density from a barium tungstate cathode.

It is a further object of this invention to provide an improved processto provide barium tungstate dispenser cathodes of increased emissiondensities at lower temperatures under continuous use.

It is another object of this invention to provide an improved thermionicdispenser type cathode comprising a sintered mixture of a refractorymetal having interspersed therein a solid solution of a pair of bariumcompounds, and operative for high emission density at lowertemperatures.

It is a further object of this invention to provide an improvedsintering and shaping process in the manufacture of barium strontiumtungstate and dispenser cathodes.

It is a further object of this invention to provide a barium strontiumtungstate dispenser type cathode for general usage which will have anemission density of in excess of at least about 5 amperes per squarecentimeter emission density at temperatures below about 1000 C. overextended periods of time.

Briefly described, this invention in one of its preferred forms includesthe combination of specific proportions of certain powdered ingredients,for example barium carbonate (BaCO strontium carbonate '(SrCO andtungsten trioxide (W0 which are intimately mixed, wet milled, dried, andfired in pellet form to provide a solid solution of barium compoundssuch as Ba WO and Ba SrWO in generally equal amounts. The pellets arethen crushed and sieved, and about 9% by weight thereof is mixed withabout 1% by weight zirconium hydride and about by weight tungsten metalpowder. The resulting mixture is dry milled, compacted under highpressure, and then subjected to high temperatures in a reducingatmosphere to fuse or melt one of the barium compounds and provide anintegral cohesive body which is formed as a high current densitythermionic dispenser type cathode.

This invention will be more fully understood when taken in connectionwith the following description and the drawing in which:

The figure is an exemplary illustration of an assembly utilizing adispenser cathode of this invention.

In the dispenser cathode prior art there exists a vast number ofdifferent processes potentially employing the same materials, which inturn provide diiferent dispenser cathodes. By the same token thedispenser cathode prior art indicates a great number of variousingredients and various proportions and mixings thereof which in turnprovide different dispenser cathode characteristics. In many of theseinstances the intermediate reaction and combinations which take place inthe production of a dispenser cathode are not known, and at the sametime, in a great number of instances those ingredients originallyemployed to provide the cathode do not appear in the final cathode inthe same form or in the same amount. Therefore, it is not readilyapparent by examination just What specific material, or combinations ofmaterials, or process steps give rise to a preferred cathode, or whichparameters may provide a greatly increased emission density.

The prior art of dispenser cathodes is indicative of braium tungstatecathodes representing a potential area from which dispenser cathodes ofsubstantially increased continuous emission densities at lowtemperatures might be obtained. It has been discovered that a bariumtungstate thermionic cathode having the constituents above described,and in the proportions given, provides a very high emission densitydisproportionate to the known reaction or contributions of theconstituent elements. In addition, this very high emission density takesplace at lower temperatures and is continuous for long life operation.Such cathodes may be produced repetitively in large numbers by followingcertain prescribed proportions and process steps.

It has also been discovered in this invention that critical items withrespect to a high emission density cathode are related as well as tothose ingredients appearing in the final item as to the beginningingredients. In addition, certain final processing may change or alterthe original ingredients to provide the final ingredients in a moredesired form sometimes irrespective of the particular amount ofparticular material available in the original form.

The basic dispenser cathode of this invention includes a porous matrixmetal body having therein a tungstate activator material and a reducingagent. The matrix body is a refractory metal, for example one or more ofthose metals taken from Groups IV, V and VI of the Periodic Table ofElements such as tungsten, molybdenum, rhenium, tantalum, etcetra, andin a preferred form of this invention is tungsten. The matrix body isfurther defined as composed of a starting material which is finelydivided or powdered high purity tungsten preferably having an averageparticle size in the range of about 2-5 microns. Particle sizes inexcess of this range provide less than optimum results in the finalproduct as based upon a series of exemplary cathodes of varying tungstenmatrix particle sizes. Powdered refractory metals, particularlytungsten, have been employed in this invention in a general range offrom about 80 weight percent to about 94 weight percent. A preferredrange is from about 88 Weight percent to about 92 weight percent withbest results from about 90 Weight percent to about 92 weight percent.

The activator material which is included in the matrix material is abarium tungstate compound, which, under operative conditions of thecathode, will be reduced and dispense barium to an active surface of thecathode. One or more barium compounds may be so employed. It has beendiscovered that best results are obtained in the practice of thisinvention when the activator starting material includes a single phasesolid solution of equimolecular amounts of Ba WO and Ba SrWO havingcubic symmetry. Peak emission density at lower temperatures is obtainedwhen the noted barium-strontium compounds are provided in substantiallystoichiometric proportions. The strontium containing tungstate has beenfound superior to other alkaline metal tungstates. Other bariumcompounds which may be employed as activator materials are BaO, Ba CaWOetcetera, which may provide tailored cathodes for specific usage wheremaximum density proportionate to lower temperature and longer life isnot the major circumstance. The activator material has been employed inthis invention in the general range of from about weight percent toabout 11 weight percent with from about 7 weight percent to about 9weight percent being a preferred range.

The reducer material employed in the dispenser cathode of this inventionperforms the function of reducing the barium compound so that barium maymigrate through the porous tungsten body to provide an active low workfunction surface. While there are various materials, elements orcompounds which may be added to the cathode to provide this function, ithas been discovered that only a limited number of refractory metalsprovide optimum results. Zirconium is one preferred reducer metal in thepractice of this invention. Zirconium should be added by way of azirconium compound such as a hydride, zirconate or oxide. Zirconiumhydride, ZrH as a preferred compound provides distinguishable results byemission density tests over the other reducer compounds. Zirconiumhydride has been employed in this invention in the general range of atleast a significant amount, i.e., an amount which will providemeasurable reduction to about 2.5 weight percent with a preferred rangebeing from about .25 weight percent to about 1.0 weight percent.Incremental additions of ZrH to standardized mixes of matrix metal andactivator compound indicate, by emission tests, that negligibleimprovement is obtained by greater amounts, and particularly,diminishing results are noted in many instances.

It has also been discovered that hafnium may be employed as a reducermetal in this invention. Hafnium may be added by way of such compoundsas oxides and hydrides, hafnium hydride HfH being preferred inproportions which may include up to about 2 weight percent. The amountof hafnium to be added should also be maintained in the same generalrange as expressed for ZrH taking into consideration the differentatomic weight involved. Other refractory metals, particularly titanium,may be employed but with distinguishable diminishing and non-equivalentresults.

In the preparation of the initial materials, very great importance isattached to proportions, particle sizes, mixing time, compressionpressures, firing or sintering temperatures and other relatedparameters, with respect to the final emission properties and life of acathode. It is a common occurrence to seriously alter an otherwiseoptimum cathode by an inadvertent or mistaken or incorrect change ofmaterials or processes or by assuming an equivalency not borne out byexperimentation.

It has been discovered that one outstanding and preferred cathode ofthis invention, i.e., one having a long life with a substantiallycontinuous emission density of about 10 amperes/cm. at a temperaturebelow about 1000 C., is provided in the specific controlled proportionsof ingredients. For example, these ingredients may comprise a generalrange of 88% to 92% by weight of refractory metal, about 7% to 9% byWeight barium compound and the balance, usually about 0.25 to 2.5% byweight of a reducer material. For a high emission density cathodeapproaching 10 or more amperes per square centimeter at 1000 C. or less,a more preferred range, and materials, are about 90% to 92% by weighttungsten, 7% to 9% by weight of a single phase solid solutioncorresponding to the general formula Ba Sr(WO obtained by a reaction ofconstituents such as for example Ba WO and Ba SrWO and from about 0.5%to 1.0% zirconium hydride. Outstanding examples include (a) about 92% byweight tungsten, about 7 /2 by weight of the described solid solutionactivator compound and about 0.5% by weight zirconium hydride, and

(b) about 90.2% by weight tungsten, about 9.2% by weight activatorcompound as described, and the balance zirconium hydride, about 0.6% byweight.

The proportions so indicated may be further described by a major percentby weight of refractory metal, i.e., 50 weight percent or more, a minorpercent by weight of an activator material, i.e., less than 50% byweight, and an additive but significant amount of a reducer materialwhich is substantially less than the amount of activator material.

The process requirements of cathodes made in accordance with theproportions above indicated are critical to the emission densitycharacteristics of the final dispenser cathode. Best results areobtained when considerable care is exercised to maintain cleanliness inconjunction with the use of high purity materials. In a preferred formof this invention, the activator material is provided by wet milling inmethanol a combination of barium carbonate (BaCO strontium carbonate(SrCO and tungsten trioxide in substantially stoichiometric proportions.After milling sufliciently to provide a very intimate mixture, themethanol is decanted and the powder material is dried and compressedinto pellets in a hydraulic press. These pellets are thereafter fired inan air or oxygen atmosphere at a temperature above 1400 C. for severalhours.

X-ray analysis of these pellets indicates a solid solution of Ba WO andBa SrWO at a composition of 50 atomic percent. Best results have beenobtained wtih this composition. However, variations are tolerable for anumber of specific applications where criteria other than or in additionto high emission rate is required. The 50 atomic percent compositiondepends on the ratio of starting materials particularly for example aratio in stoichiometric proportion such as 18.525 grams BaCO bariumcarbonate, 2.770 grams SrCO strontium carbonate and 8.705 grams W0tungsten oxide. The reaction product is or alternately 5BaO.SrO.2WO

After firing the pellets are crushed and sieved through a 325 meshscreen. This material is then mixed with tungsten which is provided infinely divided or powder form with an average particle size of 2-5microns. The proportions are in the ranges previously described,particularly specific examples (a) and (b). To this mixture there isadded a reducer such as zirconium hydride or hafnium hydried also ofabout 325 mesh or finer particle sizes, also in the preferred rangegiven, specific examples being by weight, and 0.6% respectively. Thismixture is then placed in a mixing apparatus, in this instance a smallmolybdenum ball mill, and milled for several hours. The milling time foroptimum emission is a function of the mill size, speed and charge forone particular mill.

In operative practices of this invention, ball milling was performed inan all molybdenum ball mill. The mill chamber was in the form of amolybdenum cup measuring about 2% inches diameter and 2% inches depth.The milling or grinding members were molybdenum slugs of about inchdiameter and inch length. Fifty of these milling members were employedfor each milling session or operation. A molybdenum cover plate wassecured to the open end of the mill chamber during a milling operation.During operation the mill revolved at the rate of 120 r.p.m.

Operative curves, i.e., emission density vs. milling time, of arepresentative number of these milling operations indicated a peakcondition of emission in the 2 to 6 hour milling operation range.Unexpectedly it was discovered that in extended milling operations asecond emission peak occurred in the 12 to 18 hour range. Best emissionresults were obtained in cathodes of this invention when the second peakcondition was utilized, i.e., where ball milling of the constituents wascontinued for 12 to 18 hours.

After milling, the powder mixture is then placed in a die and compactedin a hydraulic press into pellets of the general desired size.Compacting pressures contribute to the final characteristics becausethey predetermine to some extent the pore size of the finished product.In the practice of this invention compacting pressures were employed inthe range of about 40 to 90 .tons/inF. Between about 80 to 90 tons/in.provided best results.

The pressed power pellets were sintered by heating in a predeterminedatmosphere at about 1900 C. for a period of a few minutes to about 5 or6 minutes. Sintering is carried out under a relatively high purityreducing atmosphere. For example, a high purity hydrogen atmospherewhich was carefully deoxygenated and dried was employed in the preferredpractice of this invention. It was noted that the presence ofcontaminants in the hydrogen, for example water vapor and other gases,markedly affected the emission characteristics of the final cathode byreducing obtainable emission densities. The hydrogen employed had a dewpoint below about l00 F. It was further noted that the final productfrom hydrogen sintering was superior to those cathodes sintered underother atmospheres, such as insert gas atmospheres, or other atmospheresconsisting of a mixture of hydrogen and an inert gas. Additionally,comparative emission densit results indicate that the sintering processshould be carried out at temperatures in excess of about 1800 C. withoptimum results falling between 1850 and 1900 C., which is at or abovethe melting point of -Ba WO higher temperatures not indicating anyfurther favorable results.

After sintering, the cathode bodies are prepared by having the emittingsurface suitably formed or shaped to provide a fresh active surface,generally by a clean removal of a thin layer of the cathode. Bestresults were achieved when a cathode surface was carefully lathe machinewith the use of a very sharp Carboloy cutting tool similar to a facingtool. A high head stock speed, thin and slow cross feed cut is employed.It is believed that this type of cutting process exposes and/or preparesan optimum surface with excellent communication into the porous body.The pore openings of the surface are not deformed or closed by thisprocess. Other similar processes include sputtering or electronmachining. Comparative results indicate a markedly superior cathode isprovided by the machining or cutting process as compared to, forexample, an uncut surface or one obtained by grinding by differentabrasive wheels, i.e., diamond or alundum wheels.

A great number of dispenser cathodes have been produced by the describedprocess. These dispenser cathodes provided greatly increased emissiondensities at low temperatures for continuous operation, unexpected inview of the prior art knowledge of dispenser cathode tech nology.Emission density measurements were carried out in a manner tosubstantially preclude any errors in the test equipment and to provide ameasurement more reliable and precise than those indicated in ASTMstandardization test diodes for cathode evaluation. In this connection avacuum system was constructed from 347 stainless steel and bakable tothe extent of freeing the system from water vapor or absorbed gases. Allopenings were made through metal ceramic seals and compression flanges.The system is initially pumped by mercury diffusion pump because mercuryis a well defined substance that does not break down or decompose withheat and is not deleterious to cathode emission. For the higherevacuation conditions an ion pump was utilized to bring the pressure ofthe system to about 10* torr. A cathode diameter of one centimeter waschosen as being representative of sizes encountered in microwave tubes.Since cathodes of this size and made in accordance with this inventionare capable of delivering large currents, a six-phase variable voltagerectifier of 20 amperes capacity at 500 volts was constructed and usedfor all measurements. The ripple encountered in six-phase rectificationis low enough to permit the RMS values to be equivalent to straight D-Cmeasurements. Anode dissipation is one of the first problems encounteredin high current measurements. To keep anode dissipation to a minimum,diodes With 0.010 inch anode-to-cathode spacing were chosen as being awell defined compromise of electrical and mechanical considerations.Even with these close spacings, power densities of about four to sixkilowatts per square centimeter are encountered at current levelsapproaching 10 amperes. These are near maximum power concentrations thatcan be dissipated with water-cooled anodes.

A substantial number of cathodes in accordance with the practice of thisinvention were produced to better determine and define optimummaterials, material proportions, process step times and otherparameters. For example, in representative numbers of cathodes with allother parameters being maintained equal, both the kind and theproportion of given materials were varied, as well as process steps. Foreach change emission density measurements were taken to indicate theeffect of the change. Among these changes are included proportionalchange of materials, the use of other refractory metals such asmolybdenum and combinations thereof, the use of other additive materialsand other alkaline earth activator compounds, the use of other sinteringatmospheres, and the use of other active surface preparation means. Theresults of these investigations reveal that preferred cathodes are thosecontaining (a) about 92 weight percent tungsten, about 7 /2 weightpercent barium compound which includes a solid solution of Ba WO and BaSrWO and about /2 weight percent ZrH and (b) 90.2 weight percenttungsten, 9.2 weight percent solid solution and the balance zirconium orhalfnium hydride.

Furthermore, using the standard optimum proportions above described,process steps were varied over a wide range and emission densities ofthe resulting cathodes compared. The results reveal that the mostpreferred cathode includes the solid solution of Ba WO and Ba SrWO inthe activator material, sintering in a high purity reducing (hydrogen)atmosphere and very sharp clean cutting preparation of an emittingsurface.

Repetitive cathodes are accordingly produced by this invention havingcontinuous emission densities in excess of 7 to 10 amperes per squarecentimeter at operating temperatures from about 850 to 1000 C. when thepreferred compositions and production parameters as described arefollowed.

The dispenser cathodes of this invention may be utilized in a number ofdifferent assemblies and configurations. A common example is illustratedin FIG. 1. In FIG. 1 a generally cylindrical button or pill 11 of thedispenser cathode of this invention is concentrically fixed or containedwithin the end of a support cylinder 12. Button 11 is provided with anexposed emitting face surface 13 and an opposite surface 14 which isadjacent a filamentary type electrical heater 15. Button 11 may be aseparate assembly which is joined to cylinder 12 or may be formedtherein in position. Cyinder 12 is preferably a refractory material forexample molybdenum, tungsten, et cetera.

While this invention has been described with reference to particular andexemplary embodiments thereof, it is to be understood that numerouschanges can be made by those skilled in the art without actuallydeparting from the invention as disclosed, and it is intended that theappended claims include all such equivalent variations as come withinthe true spirit and scope of the foregoing disclosure.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A thermionic cathode composition consisting essentially of anintimate mixture of (a) from about 88% to 94% by weight of finelydivided refractory matrix metal,

(b) from about 5% to 11% by weight of a solid solution consisting of 50atomic percent Ba WO and 50 atomic percent Ba SrWO and (c) from about0.25% to 2.5% of a refractory metal reducer material.

2. A thermionic dispeser cathode composition consisting essentially ofan intimate mixture of (a) at least about 90% by weight of refractory ina finely divided form of from about 2 to 5 microns average particlesize,

(b) at least about 7.5% by weight of a solid solution consisting ofatomic percent Ba WO and 50 atomic percent B21 SrWO and (c) at least asmall but effective amount of a refractory metal reducer material.

3. The invention as recited in claim 2 wherein said refractory metal istungsten and said refractory metal reducer material is zirconium.

4. As an article of manufacture a sintered integral cohesive thermionicdispenser cathode consisting essentially of (a) 88 to 94 percent byweight of a porous refractory metal matrix body,

(b) 5 to 11 percent by weight of a single phasebariumstrontium-tungstate crystal of a composition corresponding to thegeneral formula Ba Sr(WO and (c) 0.25 to about 2.5 weight percent of afurther refractory metal throughout said cathode as a reducer materialfor said barium-strontium-tunstate at operative cathode temperatures,

(d) said cathode having a useful emission current density in excess of 5amperes per square centimeter at 1000" C. of continuous operation.

5. As an article of manufacture of thermionic dispenser cathodeconsisting essentially of (a) a sintered powdered tungsten body ofaverage particle size of from 1 to 5 microns which comprises from about88% to 94% by Weight of said cathode,

(b) about 7.5% by weight of a single phase solid solution consisting of50 atomic percent Ba WO and 50 atomic percent Ba SrWO dispersedthroughout said cathode, and

(c) 0.25 to 1% by weight of zirconium distributed throughout saidcathode as a reducer.

References Cited UNITED STATES PATENTS 2,848,644 8/1958 Koppius 3 l3-3462,881,512 4/1959 Huber et a1 29l82.5 3,269,804 8/1966 Affieck et al.29182.5

LELAND A. SEBASTIAN, Primary Examiner.

A. J. STEIN ER, Assistant Examiner.

US. Cl. XJR. 3l3-346

